Filtering machines of this type are described in UK 2237521 and UK 2229771 and screens of the type are described in W095/23655 and UK 2322590.
As best seen from FIGS. 1, 11 and 24 of UK 2322590 it has become conventional practice to effectively divide up the mesh area by stretching and securing layers of mesh over a frame, the large opening in which is divided into a large number of similarly sized smaller windows by an orthogonal array of ribs. The mesh is secured to the ribs as well as to the surrounding frame, so that each area of unsupported mesh is relatively small and is defined by the area of each of the small windows defined by the ribs.
Damage tends to be in the form of perforation of the mesh in one or more of the small unsupported areas of mesh, causing that area of the mesh to allow through larger particles than it should. Once this occurs the screen must be replaced or repaired.
Since each small area only accounts for a small percentage of the overall throughput of filtered material, it is possible to block a number of windows in a screen before its throughput falls too much to enable it to continue to be used. It is known to patch such windows with a layer of quick-setting epoxy or other plastics or resin based material, which when cured forms an impenetrable film or coating over the window concerned. This prevents the escape of oversize particles through the window due to the opening in the mesh caused by the damage.
It is an object of the present invention to provide an improved screen design and improved device and method for blocking off (i.e. plugging) windows in screens as aforesaid in this way.
Reference to mesh herein means woven wire mesh and the invention is equally applicable to screens having a single layer of woven wire mesh or two or more layers of woven wire mesh stretched over and secured to the frame and ribs as above described.
The invention is also applicable to frames and ribs made from plastics or reinforced plastics materials such as GRP, with or without metal reinforcing and to frames and ribs made from metal, which may be coated with a plastics or resin based material to inhibit corrosion.